1. Field of the Invention
The invention relates to a method and apparatus for controlling nitrogen oxide emissions from flue gases generated during multiple stage fluidized bed combustion, and especially during regeneration of spend FCC catalyst.
2. Description of the Prior Art
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H.sub.2, in contrast to hydrocracking, in which H.sub.2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalyst cracking (FCC) process, hydrocarbon feed contacts catalyst in reactor at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The hydrocarbons crack, and deposit some carbonaceous coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most FCc units now use zeolite-containing catalyst having high activity and selectivity. These catalysts work best when the amount of coke on the catalyst after regeneration is relatively low. It is desirable to regenerate zeolite catalyst to as low as residual carbon level as is possible. It is also desirable to burn CO completely within the catalyst regenerator system to conserve heat and to minimize air pollution. Heat conservation is especially important when the concentration of coke on the spent catalyst is relatively low as a result of high catalyst selectivity. Among the ways suggested to decrease the amount of carbon on regenerated catalyst and to burn CO in the regenerator is to add a CO combustion promoter metal in the catalyst or to the regenerator. Metals have been added as an integral component of the cracking catalyst and as a component of a discrete particulate additive, in which the active metal is associated with a support other than the catalyst.
U.S. Pat. No. 2,647,860 proposed adding 0.1-1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121, incorporated herein by reference, introduced relatively large-sized particles containing CO combustion-promoting metal into a cracking catalyst regenerator. The small-sized catalyst particles cycle between the cracking reactor and the catalyst regenerator. The large size combustion-promoting particles remain in the regenerator. Oxidation-promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated on an inorganic oxide such as alumina, are disclosed.
U.S. Pat. Nos. 4,072,600 and 4,093,535, both incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Some cracking operations using CO combustion promoters generate nitrogen oxides (NO.sub.x) in the regenerator flue gas. It is very difficult in a catalyst regeneration system to completely burn coke and CO in the regenerator without increasing the NO.sub.x content of the regenerator flue gas.
Although many refiners have recognized the problem of NO.sub.x emissions from FCC regenerators, the solutions proposed have not been completely satisfactory. The approaches taken so far have generally been directed to special catalysts which inhibit the formation of NO.sub.x in the FCC regenerator, or to process changes which reduce NO.sub.x emissions from the regenerator.
Recent catalyst patents include U.S. Pat. No. 4,300,997 and its division U.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO-combustion promoter, The bi-metallic CO combustion promoter is reported to do an adequate job of converting CO to CO.sub.2 ; while minimizing the formation of NO.sub.x.
Another catalyst development is disclosed in U.S. Pat. No. 4,199,435 which suggests steam treating conventional metallic CO combustion promoter to decrease NO.sub.x formation without impairing too much the CO combustion activity of the promoter.
Process modifications are suggested in U.S. Pat. Nos. 4,413,573 and 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NO.sub.x emissions.
U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NO.sub.x emissions.
U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel to the upper portion of a FCC regenerator to minimize NO.sub.x emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator.
U.S. Pat. No. 4,235,704 suggests too much CO combustion promoter causes NO.sub.x formation, and calls for monitoring the NO.sub.x content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NO.sub.x in the flue gas.
The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
The FCC regenerators shown in U.S. Pat. Nos. 3,893,812 and 4,197,189 are staged regenerators, which emit less NO.sub.x than FCC regenerators using a single dense bed for catalyst regeneration. It is a good regenerator, but still can produce too much NO.sub.x.
All the catalyst and process patents discussed above from U.S. Pat. Nos. 4,300,997 to 4,197,189, are incorporated herein by reference.
In addition to the above patents, there are myriad patents on treatment of flue gases containing NO.sub.x. The flue gas might originate from FCC units, or other units. U.S. Pat. Nos. 4,521,389 and 4,434,147 disclose adding NH.sub.3 to NO.sub.x containing flue gas and catalystically reducing the NO.sub.x to nitrogen.
None of the approaches described above provides the perfect solution. Process approaches which reduce NO.sub.x emissions require extensive rebuilding of the FCC regenerator.
Various catalytic approaches, eg. use of bi-metallic CO combustion promoters, provide some assistance, but the cost and complexity of a bi-metallic combustion promoter is necessary. The reduction in NO.sub.x emissions achieved by catalytic approaches helps some but still may fail to meet the ever more stringent NO.sub.x emissions limits set by local governing bodies. Much of the NO.sub.x formed is not the result of combustion of N.sub.2 within the FCC regenerator, but rather combustion of nitrogen-containing compounds in the coke entering the FCC regenerator. Bi-metallic combustion promoters are probably best at minimizing NO.sub.x formation from N.sub.2.
I have discovered a way to overcome most of the deficiencies of the prior art methods of reducing NO.sub.x emissions from a multistage FCC regenerator. The regenerators of special interest are the "minimum inventory" FCC regenerators which have a dense bed coke combustor, a dilute phase transport riser above the dense bed, and a second dense bed which holds hot, regenerated catalyst for recycle to the FCC reactor and also preferably, to the combustor.
I use conventional CO combustion promoter metals in an unconventional way. By selectively adding most of the CO combustion promoter to the transport riser, or to the top of the second regenerator dense bed I achieved a significant reduction in NO.sub.x emissions while still achieving satisfactory CO combustion. The approach was, in a sense, to turn the teaching of U.S. Pat. No. 3,808,121 upside down. The '121 patent added large-sized particles containing a CO combustion-promoting metal into an FCC regenerator. These particles because of their size and weight congregated at the bottom of the FCC regenerator dense bed. Withdrawal of hot regenerated catalyst occurred from an upper level of the FCC regenerator dense bed, so only the small-sized FCC catalyst cycled back and forth between the reactor.
In my process it is irrelevant whether or not the CO combustion promoter enters the cracking reactor, while it is essential that most coke combustion occur in a reducing atmosphere, with afterburning of CO to CO.sub.2 completed later. Preferably, most afterburning occurs in the dilute phase transport riser or in the upper portion of the second dense bed.